1. Technical Field
This disclosure relates generally to knife edge seals and, more particularly, to reducing thermal gradients within knife edge seals.
2. Background Information
Gas turbine engines include a plurality of rotor stages within both the compressor and the turbine. The rotor stages are alternately disposed with stator stages. A variety of different seal types may be used between the stages to maintain separation between high temperature core gas and lower temperature cooling gas. One type of seal often used between rotor and stator stages is a knife edge seal that includes a rotary knife edge portion and a stationary sealing land. The knife edge portion is configured to contact or be positioned in close proximity to the seal land. The knife edge portion is typically a ring-like structure with a pointed distal end and a base end that is attached to a support structure. The sealing land is typically an abradable hoop like structure having a width, which can accommodate some amount or incursion by the knife-edge portion. Knife edge seals are typically deployed to restrict fluid leakage between a region containing cooling air and an internal region containing core gas (i.e., gas that has been compressed and may include combustion products if located within the turbine).
During operation of a gas turbine engine, it is common for there to be abrupt accelerations and de-accelerations of the rotational speed of the engine. As a result, portions of the engine may be subject to relatively large transient thermal variances, resulting in different transient thermal growth patterns. For example, during a rapid acceleration, the knife edge portion of a knife edge seal can be quickly heated, relatively speaking, by an airflow traveling through the gas path. The relatively quick expansion of the knife edge portion that accompanies the increase in temperature can create significant amounts of compressive stress within the knife edge portion. The base portion of the seal, in contrast, stays relatively cool for an amount of time because the base portion is not surrounded by the same thermal input as the knife edge portion; e.g., the base portion is proximate the cooling air disposed within the internal cavity. Over time, however, the base portion and the knife edge portion can arrive at a steady-state condition if the operation of the engine remains steady-state. Conversely, if an engine decelerates from a steady-state condition, the knife edge portion may be subject to cooler core air than was present under the steady-state conditions. As a result, the knife edge portion can be exposed to another thermal variance relative to the base, wherein the knife edge portion cools more rapidly than the base portion. The relatively quick contraction of the knife edge portion that accompanies the decrease in temperature can create a significant amount of tensile stress within the knife edge portion. The stresses created by the thermal cycling can negatively affect the life expectancy of the seal.